\documentclass{standalone}
\usepackage{tikz}
\usepackage{ctex,siunitx}
\setCJKmainfont{Noto Serif CJK SC}
\usepackage{tkz-euclide}
\usepackage{amsmath}
\usepackage{wasysym}
\usetikzlibrary{patterns, calc}
\usetikzlibrary {decorations.pathmorphing, decorations.pathreplacing, decorations.shapes,}
\begin{document}
\small
\begin{tikzpicture}[>=latex,xscale=1.2]
  % \useasboundingbox(0.9,0)rectangle(5.1,5);
  \draw[<->](0,2)node[above]{$U_R$}--(0,0)--(5.2,0)node[below]{$t$};
  \draw [ thick, dashed] plot[domain=0:3.1416/2, samples=1000] function{1.5*sin(2*x)} ;
  \draw [ thick, dashed] plot[domain=3.1416:3.1416*3/2, samples=1000] function{1.5*sin(2*x)} ;
  \node at (3.1416/4,1.75) {滤波前};
  \node at (0,0)[below]{$O$};
  \draw [very thick](0,1.5*.866)--(3.1416/8, 1.5/1.414)--(3.1416/3, 1.5*.866)--(3.1416/8+3.1416, 1.5/1.414)--(3.1416/3+3.1416, 1.5*.866)--+(3.1416/8, 1.5/1.414-1.5*.866)node[right]{滤波后};
\end{tikzpicture}
\end{document}